To invert this function, let's solve the expression for x. We start with

If we subtract 6 from both sides, we have

Now we need to manipulate both sides so that the right hand side will consist only of x. To do so, we must perform the inverse function of
.
is an <em>exponential function</em>, because the variable is at the exponent (note the difference, with, for example,
, where the exponent is always 2). The inverse function of an exponential function is the <em>logarithm</em>, taken with the same base as the exponential function.
So, the inverse function of
is
. Let's apply this function to both sides to get

Which means that we can rename the variables and state that the inverse function is

i.e. option D.)